OneNote uses a local cache file to improve performance, reliability and sharing. All work in OneNote is actually done to the local cache file rather than the actual data file and then those changes are quietly sync'd to the data file in the background.
Sometimes the cache in OneNote 2007 can get corrupted and the best way to troubleshoot it is to delete it. Assuming all of the changes you have made have already been committed to the data file deleting the cache will not lose any data - but if you're at the point where you're deleting the cache file you're probably fairly desperate anyhow so a little bit of data loss may be an acceptable price to pay to get the application back on its feet.
The cache file is typically found in the: C:\Users\[username]\AppData\Local\Microsoft\OneNote\12.0 folder if you're running Vista or the C:\Documents and Settings\[username]\application data\local\Microsoft\OneNote\12.0 folder for Windows XP. With OneNote closed, using Windows Explorer go to that folder, find the OneNoteOfflineCache.onecache file and either rename it, move it or delete it. I usually recommend renaming it so that if this fix doesn't work and you decide you want to you can always rename it back.
When you start OneNote it will automatically recreate the file; hopefully
without whatever corruption caused the problem.